The present invention relates to the portable telephone communications art. Specifically, an active bias network circuit is described which provides a stable reference voltage to a radio frequency amplifier.
Portable telephone communications require a transmitter which is small and which conserves battery power. Additionally, these low power transmitters are subject to both a changing battery power supply voltage, which varies between a high charge level, after being charged, to a lower voltage level when the battery of the telephone discharges. The power amplifier of the telephone transmitter is also subject to thermal overload, due to reflected power from the antenna when it is not impedance matched to the amplifier output stage. Under reflected power conditions, a current is produced from the bipolar transistors of the amplifier which can produce a thermal runaway condition. Additionally, during operation, certain types of modulation, such as CDMA for instance, will produce a baseband modulation component which enters the bias networks for the output amplifier. Under these conditions, the baseband modulation entering the bias network circuit may and be conveyed to other circuits, producing undesirable spurious radiation components in the output power spectrum.
In order to protect the telephone transmitter from these conditions, a bias network must be provided which is isolated from the power amplification stages, and the power amplification stages must be protected against high voltage currents which would produce the thermal runaway condition. Further, these protections must be implemented so that an acceptable current drain is imposed on the battery power supply for the telephone.
A bias network for a power amplifier is provided which is isolated from baseband signals and radio frequency signals produced by the power amplifier. A current source is connected to a source of band gap voltage which produces a current proportional to the band gap voltage. A reference voltage circuit receives the current and produces a voltage proportional to the current and to the temperature of the device. The voltage is applied as a bias voltage through an operational amplifier to the power amplifier output transistors. The operational amplifier isolates the current source and reference voltage circuit from any baseband signal emanating from the power amplifier output stage. A power amplifier breakdown protection circuit is also connected to the power amplifier output stage to divert avalanche current which is produced by the power amplifier during an antenna mismatch or high supply voltage condition. Under these circumstances, the diversion of the current keeps the transistors of the power amplifier output stage from entering a thermal runaway mode.
In accordance with a preferred embodiment of the invention, a voltage regulator circuit is also connected in series to the power amplifier output stage transistors. The voltage regulator maintains the voltage to the power amplifier output transistors at a substantially constant level during a high battery voltage condition. As the supply voltage decreases, the regulator ceases operation and the full power supply voltage is applied to the output transistors.